1.

(11 + 12 + 13 + ... + 20) = ?

Answer» (112 + 122 + 132 + ... + 202) = (12 + 22 + 32 + ... + 202) - (12 + 22 + 32 + ... + 102) Ref: (12 + 22 + 32 + ... + n2) = 1 n(n + 1)(2n + 1)     6 =  20 x 21 x 41 - 10 x 11 x 21 6 6 = (2870 - 385) = 2485.


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